Removal of Organic Sulfur Pollutants from Gasification Gases at Intermediate Temperature by Means of a Zinc–Nickel-Oxide Sorbent for Integration in Biofuel Production

Abstract

Production of renewable fuels from gasification is based on catalytic processes. Deep desulfurization is required to avoid the poisoning of the catalysts. It means the removal of H2S but also of organic sulfur species. Conventional cleaning consists of a several-step complex approach comprising catalytic hydro-treating followed by H2S removal. In this work, a single-stage process using a zinc and nickel oxide sorbent has been investigated for the removal of organic sulfur species present in syngas. The process is called reactive adsorption and comes from the refinery industry. The challenge investigated by CIEMAT was to prove for the first time that the concept is also valid for syngas. We have studied the process at a lab scale. Thiophene and benzothiophene, two of the main syngas organic sulfur compounds, were selected as target species to remove. The experimental study comprised the analysis of the effect of temperature (250–450 °C), pressure (1–10 bar), space velocity (2000–3500 h−1), tar components (toluene), sulfur species (H2S), and syngas components (H2, CO, and full syngas CO/CO2/CH4/H2). Operating conditions for removal of thiophene and benzothiophene were determined. Increasing pressure and temperature had a positive effect, and full conversion was achieved at 450 °C, 10 bar and 3500 h−1, accompanied by simultaneous hydrogen sulfide capture by the sorbent in accordance with the reactive adsorption desulfurization (RADS) process. Space velocity and hydrogen content in the syngas had little effect on desulfurization. Thiophene conversions from 39% to 75% were obtained when feeding synthetic syngas mimicking different compositions, spanning from air to steam-oxygen-blown gasification. Toluene, as a model tar component present in syngas, did not strongly affect the removal of thiophene and benzothiophene. H2S inhibited their conversion, falling, respectively, to 2% and 69% at 350 °C and 30% and 80% at 400 °C under full syngas blends.